Even if civilisation and even knowledge of agriculture and domestication were lost within a few generations and we were left with no more competitive advantages over other animals, humans would still be so numerous on every continent that it would likely take thousands of years to go extinct even if there are dramatic climate changes such as an ice age.

humans would still be so numerous on every continent that it would likely take thousands of years to go extinct

This is something many doomsdayers don't seem to realize, 7.3 billion is an awfully large number. Even in many worst case scenarios of nuclear war there would still be thousands, if not hundreds of thousands, of humans spread across the Earth and in many places there would still be large enough breeding populations for humanity to survive.

Going off of our current standing it would take a fair sized or larger asteroid/comet to wipe out all life on Earth including humanity or something very rare and unlikely like multiple worst case scenarios happening at once. I have no doubts humanity will go extinct eventually, but the chances of humanity going extinct in your lifetime are pretty small. If humanity keeps going the way it is then most likely our extinction will be self inflicted, this is inevitable if we live long enough. Either through technological/biological evolution or through neglect the species we know as human today will stop existing.

This is something many doomsdayers don't seem to realize, 7.3 billion is an awfully large number. Even in many worst case scenarios of nuclear war there would still be thousands, if not hundreds of thousands, of humans spread across the Earth and in many places there would still be large enough breeding populations for humanity to survive.

Survival of humanity does not depend on numbers, but rather on social integrity. On fact, any catastrophic event will seriously afffect not only numbers, but also perception of people, who may finally consider to work together to survive in the future and go forward. Some people even thought it would be a better faith for humanity because such an event would teach them a lesson, or help to understand the importance of coexistence. But the backside of that is losing tremendous amount of knowledge and manpower, which in effect reduces ability of people to cooperate. In fact, the latter comes first - it did not took largest of empires of the past to loose a lot of population before they fell into extinction.

Unless a minor planet collides with Earth, I think the most realistic way humankind could be wiped out within a century is through a disease carefully crafted in a lab and distributed in a way aiming to wipe out humanity. The potential of biological warfare is fortunately relatively unexplored.

This reminds me of when every now and then, I'll hear Hawking make the news for saying that humanity will go extinct in about 1000 years if we don't start colonizing other worlds ("earthlike" or not). Would that be an accurate statement, or no?

it did not took largest of empires of the past to loose a lot of population before they fell into extinction.

Yet humanity still survived. There have been times in human history where global population was in the low couple thousands and we still survived. It is true that numbers are not everything but when you have as many humans as there are today the numbers lean in favor of successful breeding populations.

Quotemidtskogen ()

The potential of biological warfare is fortunately relatively unexplored.

Hate to say it, but this is not really true. Many governments and groups have invested a lot of time into researching such things and many of the results have been "positive".

The issue with making a disease to wipe out humanity lies in what is needed. First you need a disease that is not outright deadly, otherwise detection is high and treatments can usually be created and quarantines are put into effect. Then what you need is a long incubation, transmissible through air/bodily fluids, and the ability to transmit the disease during the incubation period. After that you need a way to sidestep the immune system for a while or permanently. Once you have those things down getting the disease to spread worldwide is fairly easy(this actually happens all the time in today's world). The way to spread it is fairly simple, just walk into any airport with some aerosol canisters and spray away. People will travel all over coughing, breathing, and spreading the disease.

The real difficult issues come in when you want to start killing everyone. There are bacterial infections that are immune to MOST, not all, antibiotics but the immune system in some individuals will be capable of holding off the infection so that strong antibiotics can be used or outright killing it. Perhaps some genetic tinkering could make a super bacteria but I doubt it could kill everyone. The other options you have are fungal or viral agents.

Fungal agents can be detected and screened for and in most cases treated fairly easily unless the immune system is weakened. There are some that are fairly hazardous but they require some specific environmental conditions to spread, genetic tweaking could improve this but not enough to wipe out all of humanity.

This leaves viral for the kind of disease you would want to spread in this case. The real problems come in when you start talking about what kind of virus you want to use. Using a "steady" virus that doesn't mutate much will be easy to treat and potentially cure which isn't going to get the job done. This means what you need to use is a virus that constantly mutates to evade the immune system and evade cures/treatments. However that virus then constantly mutates and simply by chance it will eventually burn itself out and mutate into something less harmful or into something the immune system can detect and fight. Even now viruses like HIV can be treated and some individuals are immune. This means even if you took the worst of the worst diseases and combined them, there will be people who will develop immunity.

There are even more issues one could list but I am sure you can see the issues with engineering such a disease. All of this means the most you could do is wipe out a significant chunk of the population but not all of humanity with a single engineered disease. You could always make a few different ones but infecting everyone or enough to kill the species would be very difficult.

The technology to do this exists today, all it is going to take is the right person to put it to use. There is also another issue, with the same technologies applied in the same fields you can develop countermeasures, treatments, and potentially cures to an engineered disease.

There is another that could potentially be used to kill humanity, that would be a prion disease. Engineering one would be exceptionally difficult and then transmitting it might be even more so. You would need to make the disease, make sure the disease can spread, remain "silent", and kill. Plenty of prion diseases are good at the killing and silent part, but most require fairly special circumstances to spread. Prions are also still not fully understood and may be caused by something like a virus so engineering a specific one is pretty much out of the question for the time being. This doesn't mean you couldn't spread one, it just means you most likely won't kill a large population of people with one. If you did solve the spreading issue, then humanity could be wiped out. There are no known cures for prion disorders and most treatments only treat the symptoms and not the disease. The chance of death from a prion infection like Creutzfeldt–Jakob(mad cow), GSS, and Fatal familial insomnia is 100% once infected. However there may be those rare few individuals who have some form of immunity to prion disorders, similar to those in Papua New Guinea who have been found to have an immunity to Kuru.

With all that, I'd be more worried about a rogue grey goo scenario than a super-bug wiping out humanity.

Yet humanity still survived. There have been times in human history where global population was in the low couple thousands and we still survived.

It is because they've had abundant resources and a lot of territory, thet is left after ther predecessors. Well, technically speaking, they probably had trouble with a lot of things, but they sustained steady growth until they filled every available territory and then even more so, after which neolithic revolution happened, changing the people forever.

The same way we might eventually encounter deficit of resources and stagnation, determined by our limited area on the planet. At some point, we may realize, it is impossible to us to continue on this path and people will be forced to stop their ruthless exploitation and suffer the consequences of their actions. We might not be able to fly into space, develop or replace infrastructure and technologies, or keep our legal system in good shape. There are some possibilities which imply different amount of changes, from minor stagnation to destruction of economy and the disappearance of nations, but what is important to know is that all of this can be avoided, if only people who are in charge would realize their arrogance and nearsightedness.